Local Group

For other uses, see Local group (disambiguation).

The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×10¹⁹ kilometres),^[1] and a total mass of the order of 2×10¹² solar masses (4×10⁴² kg).^[2] It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellites form one lobe, and the Andromeda Galaxy and its satellites constitute the other. The two collections are separated by about 800 kiloparsecs (3×10^⁶ ly; 2×10¹⁹ km) and are moving toward one another with a velocity of 123 km/s.^[3] The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.

Local Group
Local Group of galaxies, including the massive members Messier 31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
Observation data (Epoch J2000)
Parent structure	Virgo Supercluster

Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way

The two largest members, the Andromeda and the Milky Way galaxies, are both spiral galaxies with masses of about 10¹² solar masses each. Each has its own system of satellite galaxies:

• The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And V, And VI (also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph), And VII (a.k.a. the Cassiopeia Dwarf Galaxy), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.^[4]
• The Milky Way's satellite galaxies system comprises the Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy (disputed, considered by some not a galaxy), Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy), Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.^[5]

Visual size comparison of the six largest Local Group galaxies, with details

The Triangulum Galaxy (M33) is the third-largest member of the Local Group, with a mass of approximately 5×10¹⁰ M_☉ (1×10⁴¹ kg), and is the third spiral galaxy.^[6] It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart,^[7] and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk. The Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.^[8]

The other members of the group are likely gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, and Sagittarius Dwarf Irregular Galaxy.^[9]

The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy as well as Sextans B, Leo P, Antlia B and possibly Leo A, is uncertain due to extreme distances from the center of the Local Group.^[3] The Antlia-Sextans Group is unlikely to be gravitationally bound to the Local Group due to probably lying outside the Local Group's Zero-velocity surface—which would make it a true galaxy group of its own rather than a subgroup within the Local Group.^[10] This possible independence may, however, disappear as the Milky Way continues coalescing with Andromeda due to the increased mass, and density thereof, plausibly widening the radius of the zero-velocity surface of the Local Group.

History

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.^[11] There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible part of the Local Group.

Component galaxies

Clickable map

Local Group (clickable map)

List

Properties of galactic bodies in and around the Local Group^[9]

Name	Type	Constellation	Notes
Spiral galaxies
Andromeda Galaxy (M31, NGC 224)	SA(s)b	Andromeda	Largest galaxy in the group[12] Diameter (D25 isophote): 152,000 light-years Mass: (1.5±0.5)×1012 M☉ Number of stars: ca. 1012.
Milky Way Galaxy	SBbc	Sagittarius (centre)	Second largest galaxy in the group, which may or may not be the most massive galaxy of the group.[13] Diameter (D25 isophote): 87,400 light-years Mass: (1.54±0.1)×1012 M☉ Number of stars: (2.5±1.5)×1011.
Triangulum Galaxy (M33, NGC 598)	SA(s)cd	Triangulum	Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy. Diameter (D25 isophote): 60,000 light-years Mass: 5×1010 M☉ Number of stars: 4×1010.
Magellanic spiral galaxies
Large Magellanic Cloud (LMC)	Irr/SB(s)m	Dorado	Fourth largest member of the group, satellite of Milky Way and only Magellanic Spiral Galaxy in the local group Mass: 1×1010 M☉ Diameter (D25 isophote): 32,200 light-years
NGC 3109	SB(s)m	Hydra	Member of Antlia-Sextans Group
Elliptical galaxies
M32 (NGC 221)	E2	Andromeda	Satellite of the Andromeda Galaxy, shows signs of a supermassive black hole
Irregular galaxies
Wolf–Lundmark–Melotte (WLM, DDO 221)	Ir+	Cetus	Possible size between Small Magellanic Cloud and Large Magellanic Cloud
IC 10	KBm or Ir+	Cassiopeia
Small Magellanic Cloud (SMC, NGC 292)	SB(s)m pec	Tucana	Satellite of Milky Way, 6th largest galaxy in the local group mass: 7 × 109 M☉
Pisces Dwarf (LGS3)	Irr	Pisces	Possible satellite of the Triangulum Galaxy
IC 1613 (UGC 668)	IAB(s)m V	Cetus
Phoenix Dwarf	Irr	Phoenix
Leo A (Leo III)	IBm V	Leo
Aquarius Dwarf (DDO 210)	IB(s)m	Aquarius	Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999.[14]
SagDIG (Sagittarius Dwarf Irregular Galaxy)	IB(s)m V	Sagittarius	Most remote from barycenter member thought to be in the Local Group.[14]
NGC 6822 (Barnard's Galaxy)	IB(s)m IV-V	Sagittarius
Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216)	Irr	Pegasus
UGC 4879 (VV124)	IAm	Ursa Major	One of the most isolated galaxies in Local Group. Situated at the edge of the Local Group.
Sextans A (UGCA 205)	Ir+V	Sextans	Member of Antlia-Sextans Group
Sextans B (UGC 5373)	Ir+IV-V	Sextans	Member of Antlia-Sextans Group
Leo P	Irr	Leo	Member of Antlia-Sextans Group, extraordinarily low metallicity (Z = 0.03ZMW)
AGC 198606	Irr?	Leo	Gas-rich ultra-faint dwarf galaxy
AGC 215417	Irr?	Leo	Gas-rich ultra-faint dwarf galaxy
AGC 219656	Irr?	Leo	Gas-rich ultra-faint dwarf galaxy
AGC 249525	Irr?	Boötes	Gas-rich ultra-faint dwarf galaxy, Situated at the edge of the Local Group
AGC 268069	Irr?	Serpens	Gas-rich ultra-faint dwarf galaxy
Dwarf elliptical galaxies
M110 (NGC 205)	dE6p	Andromeda	Satellite of the Andromeda Galaxy and 5th largest galaxy with the mass of 9.3 billion solar masses.
NGC 147 (DDO 3)	dE5 pec	Cassiopeia	Satellite of the Andromeda Galaxy
Dwarf spheroidal galaxies
Boötes I	dSph	Boötes
Cetus Dwarf	dSph/E4	Cetus
Canes Venatici I Dwarf and Canes Venatici II Dwarf	dSph	Canes Venatici	Satellites of the Milky Way[15][16]
Andromeda III	dE2	Andromeda	Satellite of the Andromeda Galaxy
NGC 185	dE3 pec	Cassiopeia	Satellite of the Andromeda Galaxy
Andromeda I	dE3 pec	Andromeda	Satellite of the Andromeda Galaxy
Sculptor Dwarf (E351-G30)	dE3	Sculptor	Satellite of Milky Way
Andromeda V	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda II	dE0	Andromeda	Satellite of the Andromeda Galaxy
Fornax Dwarf (E356-G04)	dSph/E2	Fornax	Satellite of Milky Way
Carina Dwarf (E206-G220)	dE3	Carina	Satellite of Milky Way
Leo I (DDO 74)	dE3	Leo	Satellite of Milky Way
Sextans Dwarf	dE3	Sextans	Satellite of Milky Way
Leo II (Leo B)	dE0 pec	Leo	Satellite of Milky Way
Ursa Minor Dwarf	dE4	Ursa Minor	Satellite of Milky Way
Draco Dwarf (DDO 208)	dE0 pec	Draco	Satellite of Milky Way
SagDSG (Sagittarius Dwarf Spheroidal Galaxy)	dSph/E7	Sagittarius	Satellite of Milky Way
Tucana Dwarf	dE5	Tucana
Cassiopeia Dwarf (Andromeda VII)	dSph	Cassiopeia	Satellite of the Andromeda Galaxy
Pegasus Dwarf Spheroidal Galaxy (Andromeda VI)	dSph	Pegasus	Satellite of the Andromeda Galaxy
Ursa Major I Dwarf	dSph	Ursa Major	Satellite of the Milky Way
Ursa Major II Dwarf	dSph	Ursa Major	Satellite of the Milky Way
Ursa Major III	dSph	Ursa Major	Satellite of the Milky Way
Leo IV	dSph	Leo	Satellite of the Milky Way
Leo V	dSph	Leo	Satellite of the Milky Way
Leo T	dSph/Irr	Leo	Satellite of the Milky Way
Boötes II	dSph	Boötes	Satellite of the Milky Way
Boötes III	dSph	Boötes	Satellite of the Milky Way
Boötes IV	dSph	Boötes	Satellite of the Milky Way
Coma Berenices	dSph	Coma Berenices	Satellite of the Milky Way
Segue 2	dSph	Aries	Satellite of the Milky Way
Hercules	dSph	Hercules	Satellite of the Milky Way
Pisces II	dSph	Pisces	Satellite of the Milky Way
Reticulum II	dSph	Reticulum	Satellite of the Milky Way
Reticulum III	dSph	Reticulum	Satellite of the Milky Way
Eridanus II	dSph	Eridanus	Probable satellite of the Milky Way
Grus I	dSph	Grus	Satellite of the Milky Way
Grus II	dSph	Grus	Satellite of the Milky Way
Tucana II	dSph	Tucana	Satellite of the Milky Way
Hydrus I	dSph	Hydrus	Satellite of the Milky Way
Draco II	dSph	Draco	Satellite of the Milky Way
Carina III	dSph	Carina	Satellite of the Milky Way
Triangulum II (Laevens 2)	dSph	Triangulum	Satellite of the Milky Way
Carina II	dSph	Carina	Satellite of the Milky Way
Pictor II	dSph	Pictor	Satellite of the Milky Way
Horologium II	dSph	Horologium	Satellite of the Milky Way
Virgo I	dSph	Virgo	Satellite of the Milky Way
Virgo III	dSph?	Virgo	Satellite of the Milky Way[17]
Sextans II	dSph?	Sextans	Satellite of the Milky Way[17]
Aquarius II	dSph	Aquarius	Satellite of the Milky Way
Crater II	dSph	Crater	Satellite of the Milky Way
Hydra II	dSph	Hydra	Satellite of the Milky Way
Antlia II	dSph	Antlia	Satellite of the Milky Way
Pegasus III	dSph	Pegasus	Satellite of the Milky Way
Pegasus IV	dSph	Pegasus	Satellite of the Milky Way[18]
Pegasus W	dSph	Pegasus	Recent star formation, could still be starforming
Cetus III	dSph	Cetus	Satellite of the Milky Way
Leo K	dSph?	Leo	Satellite of the Milky Way[19]
Leo M	dSph?	Leo	Satellite of the Milky Way[19]
Leo Minor I	dSph?	Leo Minor	Satellite of the Milky Way[20]
Boötes V	dSph?	Boötes	Satellite of the Milky Way[20]
Virgo II	dSph?	Virgo	Satellite of the Milky Way[20]
Tucana B	dSph	Tucana	[21]
Perseus I Dwarf Galaxy (Andromeda XXXIII)	dE	Perseus	Satellite of Andromeda
Antlia Dwarf	dE3/dSph/Irr?	Antlia	Member of Antlia-Sextans Group
Andromeda IX	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda X	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XI	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XII	dSph	Andromeda	Possible satellite of the Andromeda Galaxy
Andromeda XIII (Pisces III)	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XIV (Pisces IV)	dSph	Pisces	Possible satellite of the Andromeda Galaxy
Andromeda XV	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XVII	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XIX	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XX	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXI	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXII	dSph	Pisces	Possible satellite of the Triangulum Galaxy
Andromeda XXIII	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXIV	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXV	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXVI	dSph	Andromeda	Satellite of the Andromeda Galaxy
Andromeda XXVII	dSph	Andromeda	Satellite of the Andromeda Galaxy, tidally disrupted[22]
Andromeda XXIX	dSph	Pegasus	Satellite of the Andromeda Galaxy
Andromeda XXX (Cassiopeia II)	dSph?	Cassiopeia	Satellite of the Andromeda Galaxy
Andromeda XXXI (Lacerta I)	dSph?	Lacerta	Satellite of the Andromeda Galaxy
Andromeda XXXII (Cassiopeia III)	dSph?	Cassiopeia	Satellite of the Andromeda Galaxy
Andromeda XXXIV (Pegasus V)	dSph	Pegasus	Satellite of the Andromeda Galaxy[23]
Andromeda XVI (Pisces V)	dSph	Pisces	Possible satellite of the Andromeda Galaxy
Andromeda XXVIII	dSph?	Pegasus	Possible satellite of the Andromeda Galaxy
Andromeda XXXIII (Perseus I)	dSph?	Perseus	Possible satellite of the Andromeda Galaxy
Andromeda XVIII	dSph	Andromeda
Centaurus I	dSph	Centaurus	Satellite of the Milky Way[24]
Pisces VII (Triangulum III)	dSph?	Pisces	Candidate, possible satellite of the Triangulum Galaxy[25]
Identification unclear
Virgo Stellar Stream	dSph (remnant)?	Virgo	In the process of merging with the Milky Way
Canis Major Dwarf	Irr?	Canis Major	Possibly a dwarf galaxy in the process of merging with the Milky Way
Hydra 1		Hydra	Possibly a dwarf galaxy in the process of merging with the Milky Way
Tucana III	dSph or cluster?	Tucana	Satellite of the Milky Way, tidally disrupting
Tucana IV	dSph or cluster?	Tucana	Satellite of the Milky Way
Tucana V	dSph or cluster?	Tucana	Possibly non-existent
Columba I	dSph or cluster?	Columba	Satellite of the Milky Way
Segue 1	dSph or Globular Cluster	Leo	Satellite of the Milky Way
Cetus II		Cetus	Likely part of Sagittarius tidal stream
Willman 1	dSph or Globular Cluster	Ursa Major	147,000 light-years away
Horologium I	dSph or Globular Cluster	Horologium	Satellite of the Milky Way. Not to be confused with the Horologium Supercluster.
Pictoris	dSph or Globular Cluster	Pictor	Satellite of the Milky Way
Phoenix II	dSph or Globular Cluster	Phoenix	Satellite of the Milky Way
Indus I (Kim 2)	dSph or Globular Cluster	Indus	Satellite of the Milky Way
Eridanus III	dSph or Globular Cluster	Eridanus	Satellite of the Milky Way or SMC[26]
Sagittarius II	dSph or Globular Cluster	Sagittarius	Satellite of the Milky Way
Andromeda VIII	dSph?	Andromeda	Satellite of the Andromeda Galaxy, tidally disrupting
Antlia B		Antlia	Member of Antlia-Sextans Group
Probable non-members
Andromeda IV	Irr	Andromeda	Once considered to be associated with M31. Its distance is now known to be 22 to 24 million light years (not close to the Andromeda Galaxy at all).[27]
GR 8 (DDO 155)	Im V	Virgo	Distance 7.9 million light years[28]
IC 5152	IAB(s)m IV	Indus	Distance 5.8 million light years, possibly an outlying member of the local group[29]
NGC 300	SA(s)d	Sculptor	Distance 6.07 million light years
NGC 55	SB(s)m	Sculptor	Distance 6.5 million light years[30]
NGC 404	E0 or SA(s)0−	Andromeda	Distance 10 million light years[31]
NGC 1569	Irp+ III-IV	Camelopardalis	In IC 342 group of galaxies. Distance 11 million light years[32]
NGC 1560 (IC 2062)	Sd	Camelopardalis	Distance 8-12 million light years
Camelopardalis A	Irr	Camelopardalis	Distance 12 million light years[30]
Argo Dwarf	Irr	Carina
ESO 347-8 (2318–42)	Irr	Grus
UKS 2323-326 (ESO 407-18)	Irr	Sculptor	Distance 7.2 million light-years[30]
UGC 9128 (DDO 187)	Irp+	Boötes
KKs 3	dSph	Hydrus
Objects in the Local Group no longer recognised as galaxies
Palomar 12 (Capricornus Dwarf)		Capricornus	Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 4 (Ursa Major Dwarf)		Ursa Major	Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 5 (Serpens Dwarf)		Serpens	Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 3 (Sextans C)		Sculptor	Globular cluster formerly classified as a dwarf spheroidal galaxy[33]
Segue 3		Pegasus	Globular cluster formerly classified as a dwarf spheroidal galaxy[34]
Laevens 1 (Crater Dwarf)		Crater	Globular cluster formerly classified as a dwarf spheroidal galaxy[35]
DES J2038-4609 (Indus II)		Indus	Likely a chance alignment of stars[36]
Name	Type	Constellation	Notes

Structure

Overview of the structure and trajectory of the Local Group

Streams

• Magellanic Stream, a stream of gas being stripped off the Magellanic Clouds due to their interaction with the Milky Way
• Monoceros Ring, a ring of stars around the Milky Way that is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy

Future

The galaxies of the Local Group are likely to merge together under their own mutual gravitational attractions over a timescale of tens of billions of years into a single elliptical galaxy, with the coalescence of Andromeda and the Milky Way being the predominant event in this process.^[37]

A NASA conception of the collision using computer-generated imagery

Location

Main article: Laniakea Supercluster

This paragraph is an excerpt from Laniakea Supercluster.[edit]

The Laniakea Supercluster (/ˌlɑːni.əˈkeɪ.ə/; Hawaiian for "open skies" or "immense heaven")^[38] or the Local Supercluster (LSC or LS) is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies.

A diagram of our location in the observable universe. (Alternative image.)

See also

• Galaxy cluster
• IC 342/Maffei Group, possibly the group of galaxies nearest to the Local Group
• Antlia-Sextans Group, another candidate for the group of galaxies nearest to the Local Group
• List of galaxy groups and clusters
• List of nearest galaxies
• Virgocentric flow

References

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2. The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy. Estimates for the mass of each galaxy are compatible with 10¹² M_☉, and Peñarrubia et al. (2014) estimate (2.3±0.7)×10¹² M_☉ for the Local Group, but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of (1.29±0.14)×10¹² M_☉.
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External links

• "The Local Group". www.messier.seds.org. Retrieved 12 March 2020.
• "Local Group Galaxies Survey". www2.lowell.edu. Retrieved 12 March 2020.
• van den Bergh, Sidney (2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.